1. Field of the Invention
The present invention relates to a capacitance-type electrostatic transducer having a backplate in engagement with a vibratile diaphragm with the center of said backplate being subjected to a force, in general, and to such a transducer backplate whose center is subjected to a diaphragm tensioning force from a leaf spring, in particular.
2. Description of the Prior Art
Capacitance-type electrostatic transducers are well-known in the prior art. In such transducers, a diaphragm having an insulative layer and an electrically conductive layer has its insulative layer in contact with a grooved, irregular, electrically conductive backplate surface. A peripheral region of the diaphragm is maintained in a fixed position with respect to the transducer housing and a spring force urges said grooved backplate surface into cooperative engagement with said insulative diaphragm layer.
The insulative and electrically conductive layers of the diaphragm and the conductive surface of said backplate form a capacitor such that when a dc voltage is applied across the electrodes of said capacitor, irregularities in the grooved surface of the backplate set up localized concentrated electric fields in said insulative layer. When an ac signal is superimposed on said dc bias, the insulative layer is stretched such that oscillatory formations develop, causing an acoustical wavefront to be propagated from the diaphragm. The received acoustical wavefront impinging upon the insulative layer produces a variable voltage corresponding to said impinging wavefront across the capacitor electrodes.
The use of a single transducer for both signal transmission and signal reception necessitates desensitizing or "blanking" of receive signal circuitry during signal transmission in order to prevent said receive signal circuitry from confusing a transmit signal with an echo of said transmit signal. Such a desensitizing or "blanking" arrangement is described in U.S. Pat. No. 4,199,246, by J. MUGGLI.
An extremely important design consideration for the above-described type of transducer is obtaining the proper diaphragm tension. Diaphragm tensioning greatly influences transducer acoustical output magnitude and direction, reception sensitivity and resonant frequency, for example. The prior art discloses several arrangements for obtaining the desired amount of diaphragm tensioning.
In one such diaphragm tensioning arrangement described in U.S. Pat. No. 4,085,297 to PAGLIA, a transducer backplate is urged into cooperative engagement with a vibratile diaphragm by a leaf spring that applies a force to the center region of said backplate in order to achieve said cooperative engagement. A leaf-type diaphragm tensioning spring is low in cost, of minimum size, is easily fabricated and is readily assembled on a transducer body.
A problem believed to be associated with the use of a leaf spring or a spring arrangement in an electrostatic transducer where the diaphragm tensioning force is applied to the center region of a cooperating backplate, is the generation of spurious signals by said transducer due to the fact that the peripheral region of said backplate is cantilevered or is unsupported by a spring or any other force producing means. If such signals should occur after the receive circuitry is made sensitive to an echo of a previously transmitted signal, and these spurious signals are sensed by the transducer before it senses a true echo of said previously transmitted signal, these spurious signals will cause a false signal to be produced at the input to said receive circuitry that will appear to be an actual or true echo of said previously transmitted signal. This false signal may introduce an error into a system that utilizes an electrostatic transducer for both signal transmission and signal reception and also relies on the flight time from when a signal is transmitted and an echo of said signal is sensed by said transducer.
In U.S. patent application Ser. No. 101,425, filed Dec. 10, 1979, by R. PAGLIA, the applicant of the present invention, a capacitance-type electrostatic transducer is disclosed wherein the backplate and vibratile diaphragm of said transducer are urged into cooperative engagement with one another by means of a resilient spring and a force transferring disc having a circular raised portion at its periphery, interposed between said backplate and said spring. The raised portion of said disc makes circular backplate supporting contact with the periphery of said backplate to thereby substantially reduce unwanted backplate vibrations. While this particular disc/spring arrangement is effective for reducing backplate vibrations and improving diaphragm/backplate contact, it has certain undesirable aspects that increase the overall cost of a complete transducer assembly. For example, it takes more material and time to fabricate said disc/spring members than it takes to fabricate a single spring member. Also, additional labor is required in order to assemble said disc member on a transducer body.